The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus which is provided with a plurality of process units which record images on a recording medium such as a continuous recording paper using the electrophotography technique.
When recording images on both sides of the recording paper using a single process unit, the recording paper is turned over after recording the image on a first side of the recording paper, and the image is thereafter recorded on a second side of the recording paper. For this reason, there is a limit to increasing the image recording speed, and a method has been proposed to use two process units for respectively recording the images on the first and second sides of the recording paper.
On the other hand, when recording a color image, there is a method which uses a single process unit. In this case, the single process unit is used to successively record images of each of the colors such as yellow, magenta, cyan and black on the recording paper in an overlapping manner, and there is also a limit to increasing the image recording speed. Hence, a method has been proposed to provide a plurality of process units, and to record the image of each color using one of the process units.
Conventionally, in a printer which uses the electrophotography technique and records the image on the continuous recording paper, if one print operation (job) ends and a next job does not occur for a predetermined time, a portion of the paper printed with the last line remains within a paper transport path within the printer. Accordingly, the recording paper is fed forward manually or automatically so as to eject the portion of the paper printed with the last line outside the printer, so that the recording paper can be cut along a perforation provided subsequent to the portion of the paper printed with the last line.
However, when the next job is started in this state, the printing is started in a state where the amount of recording paper which is fed forward to remove the print result of the previous job precedes the first line of this next job. As a result, the amount of the recording paper which is fed forward is wasted, and the utilization efficiency of the recording paper is poor.
Accordingly, a so-called back-feed is conventionally carried out if one job ends and the next job does not occur for a predetermined time. This back-feed feeds the recording paper forward if one job ends but the next job does not occur for the predetermined time, and returns the recording paper by a predetermined amount before the next job starts. As a result, the utilization efficiency of the recording paper is improved. A photoconductive body of the process unit and the recording paper are separated from each other when carrying out the back-feed.
Similarly, in the case of the printer which successively records images by the first and second process units, for example, the last line that is recorded is located on a downstream side of the second process unit in a transport direction of the recording paper at a point in time when one job ends. Hence, if the next job starts in this state, the portion of the paper between the first and second process units is wasted. For this reason, a method has been proposed to also carry out the back-feed described above in such a case.
However, in the case of the printer which successively records the images by the first and second process units, if a single fixing unit is used to simplify the printer construction, this fixing unit is provided on the downstream side of the second process unit which is provided on the most downstream side along the transport direction of the recording paper. Consequently, when the above described back-feed is carried out, the image recorded on the recording paper by the previous job is not yet fixed. Therefore, when the photoconductive body of each process unit and the recording paper make contact at the start of the next job, there was a problem in that the image which is not yet fixed may be disturbed by the shock of the contact between the photoconductive body and the recording paper. If the image which is not yet fixed is disturbed, the image recording quality greatly deteriorates and it becomes necessary to carry out the two jobs over again, thereby introducing another problem in that the performance of the printer greatly deteriorates.